Analogue-to-digital converters (ADCs) are known and used in a variety of applications. One particular application is within audio signal paths. Increasingly audio data is being stored and transmitted in digital format. An ADC may therefore be arranged in an audio signal path to convert an analogue signal into an equivalent digital signal, i.e. digital data, which may be stored or passed on for further processing.
One particular application where ADCs are used for converting analogue audio signals detected by a microphone into a corresponding digital signal is in, for example, a portable communications device such as a mobile telephone. Increasingly such microphones are required to have a relatively large dynamic range to cope with issues such as large-amplitude interfering signals such as wind noise. Consequently the ADCs are also required to have a large dynamic range.
In such applications, to maximize the effective resolution of the ADC, it is known to apply a signal-dependent analogue gain (GIN) to the input analogue signal prior to digital conversion and to apply a corresponding inverse digital gain (GO=1/GIN) adjustment to the converted digital signal in order to compensate for the applied analogue gain change GIN. This is known as dynamic range extension (DRE). Typically, a small amplitude analogue input signal may be amplified by a large value of GIN so as to make use of more of the ADC input range, thus effectively improving the resolution of the ADC for small amplitude signals. A correspondingly large digital attenuation is applied to the digital signal to compensate for the large analogue amplification. This means that, for small signals, the quantization noise and thermal noise of the ADC may be attenuated by the low digital gain (GO=1/GIN).